1. Field of the Invention
The present invention relates to strung rackets for hitting compressible balls, and more particularly relates to stringing networks in tennis rackets for stroking balls more effectively.
2. Description of the Prior Art
In the sport of tennis, a ball is hit from one side of the court to the other, the sides being separated by a net. The ball is usually a compressible ball covered by a fabric or fabric-like outside surface. The ball is stroked by a racket which comprises a handle connected through the neck of the racket to a stringing frame, which is usually oval or elliptical in shape. Many other shapes, however, such as circular frames, rhomboidal frames, pear-shaped frames, etc. have been employed at various times during the last 100 years. (See U.S. Pat. No. 2,059,917 to Spencer.)
The area within the closed or substantially closed stringing frame is strung with gut, nylon or similar string capable of having a very high tension. It has been customary to string the entire area within the racket frame with a single gut or string, which is threaded through holes in the frame and woven through and around the other string portions of the stringing network which are encountered during each traverse across the frame. The most common form of stringing within the frame includes a series of spaced vertical string traverses or string crossings, crossing from the top of the frame to the bottom or neck of the frame, and a plurality of spaced horizontal string crossings crossing transversely from one side of the racket frame to the opposite side of the racket frame. The entire stringing may be called a network or a stringing network.
In this application, the term "string" will at times, which will be clear from context, be used as a shortened form of the term "string crossing", to denote a string crossing from one side of the racket frame to the oppositely disposed side of the racket frame. The term "string" when so used should not be understood to indicate a separate, individual string, since, as noted above, the entire stringing network is frequently strung with a single string. Rather, when so used, the term "string" should indicate only a string crossing as defined.
In stringing a racket, the string is pulled taut, to provide a predetermined tension commensurate with the strength of the frame. Very expensive and exotic materials, such as graphite or boron fibered plastics or fiberglass, have been used for the frame, and often times for the entire racket in an effort to limit weight and yet increase the strength of the frame enough to support very high tensions as required in conventional rackets for very hard hitting.
The network is ordinarily threaded so that each transverse string crossing is woven first on top and then underneath successive vertical strings which are crossed in the stringing process. Conventionally, it has been customary to have the spacing between adjacent parallel strings, either in the vertical or in the transverse directions, equal so as to provide a uniform appearing surface throughout the stringing network area within the frame. In such conventional stringing, the parallel equi-spaced horizontal strings are not extended into the very top and bottom areas of the racket.
The appearance and dimensioning of a conventionally strung tennis racket is shown and described in U.S. Pat. No. 4,013,289 and can be considered to comprise a network of 18 vertical (main) strings and approximately 20 horizontal (cross) strings interwoven at substantially 7/16" spacings to form substantially 7/16" squares.
A conventionally strung tennis racket has very serious deficiencies. Such a racket has a rather small "sweetspot." The sweetspot is that area of the stringing network, usually centered just below the center of the frame, which will give very good response when the ball is impacted thereon. It is the area of the racket face at which players report that the shot "feels good" when the ball is hit there. When the ball is hit in the sweetspot area, the player perceives little or no jolt of the racket as he hits the ball and follows through to complete the stroke. The sensation is of a continuous stroke in which the racket and the ball join for a time and then the ball is eventually allowed to depart from the racket at a time and direction which appears to the player to be very controllable and predictable. Moreover, when the ball is hit on the sweetspot, the perception of the player is often that he has ample time and control to apply any desired type of spin, as for example, top spin or slice, etc., which have such important strategic significance in tennis. In a conventional tennis racket, all of the good features of hitting on the sweetspot drop off dramatically as the ball is hit away from the sweetspot. Jar is increased, control and predictability of ball direction are increasingly diminished, and the ability to apply controllable spin is increasingly diminished. These deficiencies, as you hit away from the sweetspot, are expecially severe as the point of impact moves upwards towards the top of the racket and culminates in a large "dead spot" at the top of the racket. If the ball is hit near the top-of-the-racket dead spot, it feels very much like you have hit the racket with a wooden board rather than with a set of strings and power and velocity and directionality are lost. This top-of-the-racket dead spot causes especially severe difficulties in the tennis serve, and in part accounts for the enormous differences in the ability of champion players and just ordinary good players to deliver a powerful tennis serve. The very best tennis players hit the serve very close to the sweetspot. All other players tend to hit the ball in the serve well above the sweetspot and even into the margins of the top-of-the-racket dead spot, thus creating enormous differences in the effectiveness of serving.
There are other very serious disadvantages of the conventionally strung racket. If very high hitting power is desired, the tennis racket will be strung to a very high tension to increase the "stiffness" of the stringing network. However, only very good tennis players can use a really tightly strung racket because there is a tendency for a very rapid rebound of the ball from the racket, and therefore decrease of dwell time of the ball on the racket and hence loss of directionality and control, unless the player is very skilled in applying his stroke. A very good tennis player not only "follows through" on his stroke but actually accelerates the racket head in a very predetermined manner during the driving portion of the stroke in order to keep the racket in contact with the ball throughout such period of the stroke and thereby increases directionality and control as the ball adopts the direction in which the racket is moved. (Without such prolonged contact between racket and ball, the direction of rebound of the ball tends to be more determined by the laws of mirror reflection from the plane surface of the racket.) Thus, a very good player can compensate for the loss of directionality and control which is inherent in a very tightly strung racket. Even a very good player, however, pays a price for a very tightly strung racket because (1) very strong and expensive frames must be used to support the high string tensions, (2) the high string tension reduces string life, (3) the racket tends to be very unforgiving when the player makes an occasional poor or even marginal shot, and (4) the attention required for control of a very highly strung racket is a drain upon the attention that might otherwise be available for such high level and very important matters as anticipation of the opponent, footwork, and general court strategy.
Most players who are not truly expert, find that they can play best with a "softer," lower-tension, more forgiving stringing which has a somewhat larger sweetspot which will give them inherently lower hitting power but greater controllability and directionality and predictability. But such a compromise also has its prices. If power is lower at the sweetspot, it will tend to be very much lower off the sweetspot, so that the average player tends to make many weak returns. The top-of-the-racket dead spot remains as a dangerous hazard in serving. String life is reduced by another mechanism, by increasing sawing of the strings upon each other due to their relative looseness and movability when the ball is impacted. Also, the sweetspot although somewhat enlarged remains still quite small, with great loss of power, control, predictability, etc., as the less skilled, average player hits frequently off the sweetspot throughout play.
In addition, all tennis rackets, whether tightly strung or more softly strung, have very serious limitations in their ability to apply and control spin to the ball because of the low frictional contact between the ball and the strings as well as the limited dwell time of the ball on the racket.
There have been many prior art nonconventional tennis racket stringings that have been utilized in attempt to remedy some of the above-described deficiencies of the conventional racket. In general, these prior art nonconventional stringings are somewhat irrational and reflect an incomplete or even incorrect understanding of the determining factors of string performance. Thus, for example, one prior art racket (U.S. Pat. No. 323,608) has extra horizontal cross strings across the top to supposedly strengthen and reinforce the dead spot at the top of the racket. However, as will be later explained, such horizontal tennis strings at the top of the racket, because of their extreme shortness, would be expected to contribute to deadness at the top of the racket rather than to eliminate deadness at that point.
Another prior art approach with a somewhat atypical or unconventional stringing pattern is described in the U.S. Pat. No. 4,013,289, by Kaminstein, which purports to enlarge the sweetspot of a conventional racket by decreasing the number of strings at the margin of the racket. The approach of the Kaminstein device is rational though incomplete. It does provide some of the features which would be obtained from an enlarged sweetspot. Because of the moderate trampolining effect of this stringing, the ball can more readily stay in contact longer with the central areas of the racket and will, when propelled from the racket by the rebounding strings, depart in softly hit returns with greater power from the racket than would be otherwise obtained. However, as with all trampolining systems, there are also substantial difficulties. If the racket stringing is soft, the trampolining effect may be excessive, making the time and velocity of departure of the ball unpredictable. If the stringing is tight, because of the reduced number of strings (relative to the conventional racket) at the margins of the racket, very large forces will be applied to these strings if the ball hits at the margin, and they may break or respond in an erratic manner. The severe dead spot at the top of the racket remains. Also, at the sides of the racket, because of the reduced number of vertical strings that will make contact with the ball if the ball is hit at either side, both directionality and friction is impaired, thus reducing the ability to control direction and spin of a marginally hit ball. Thus the Kaminstein device is viewed as a generally rational but incomplete approach to the solution of the problems described above.
What is desired is to provide a tennis racket that has a stringing network that meat can be adapted to give a greatly enlarged sweetspot, precise diminution of dead spots, especially at the top of the racket, reduced string wear, controllable areal stiffness for hard hitting, prolonged dwell time of the ball on the racket, and increased frictional contact between ball and racket for improved application and control of spin to the ball.